1. Field of the Invention
The present invention is directed to a heart pacemaker, and in particular to a heart pacemaker wherein ventricular stimulation occurs following the sensing of a natural atrial contraction, and which includes a sensor for a physiological parameter indicative of the physical activity of the pacemaker user and which uses the sensed parameter as a basis for limiting the rate of ventricular stimulation. A corresponding method is also disclosed.
2. Description of the Prior Art
A heart pacemaker commercially available from Siemens-Elema AB of Solna, Sweden is described in the publication "Pulse Generator 704 - Physician's Manual" (March 1985). This pacemaker can operate in a number of different modes, including the DDD mode. In this operating mode, a heart muscle contraction in the region of the ventricle is stimulated, if needed, after the detection of a spontaneous (i.e., natural or non-pacemaker stimulated) heart muscle contraction in the region of the atrium. Ventricular stimulation occurs if no natural heart muscle contraction in the region of the ventricle appears during a time span, known as the A-V interval, which follows the detection of the spontaneous heart muscle contraction in the region of the atrium. In this operating mode, a heart muscle contraction in the region of the atrium is stimulated when a second time interval, referred to as the base interval, has elapsed without a spontaneous heart muscle contraction being detected in the region of the atrium. The generation of an atrial stimulation pulse activates both the base interval and the A-V interval. The repetition rate of the ventricular stimulations is limited to a maximum value, known as the highest synchronous rate (HSR). The highest synchronous rate is approximately 150 stimulations per minute.
The DDD operating mode is prescribed for various types of cardiac pathology, particularly for patients having A-V block. The DDD mode, however, is not utilized for patients who suffer fibrillations of the atrium which cannot be treated with medication. For these patients there is the risk that the heart muscle contractions which appear during fibrillation of the atrium will be detected and which, in view of the high repetition of the heart muscle contractions in the atrium during fibrillation, would result in the heart ventricle being artificially stimulated at a repetition rate which corresponds to the maximum value, i.e., to the HSR. In patients wherein the risk of atrial fibrillation is present, a conversion is generally made to the VVI operating mode, in which both the detection and stimulation of ventricular contractions occurs, with a ventricular contraction always being stimulated if no natural ventricular contraction is detected within a defined time interval.
In some patients, atrial fibrillation occurs intermittently. Nonetheless, the pacemaker must operate in the VVI mode for these patients, even though it would be desireable to permit the pacemaker to operate in the DDD mode as long as there is no atrial fibrillation.
In theory it is conceivable to detect atrial fibrillation and automatically to switch the heart pacemaker, normally operating in the DDD mode, to the VVI mode only during the appearance of fibrillation in the atrium. This conceptual approach, however, has significant practical problems. First, it is difficult to obtain the necessary faultless intracardial electrocardiogram of atrial activity which would be needed to accurately trigger the switch from the DDD mode to the VVI mode. Second, even if such an accurate atrial electrocardiogram could be obtained, an algorithm to identify atrial fibrillation would be needed, because of the pronounced changes in amplitude and large fluctuations in the heart rhythm which occur during atrial fibrillation could not be handled without clipping and/or distortion by conventional pacemaker components.
The same problems are present for other atrial synchronous operating modes of pacemakers.